Nitroaromatic compositions have been widely used as intermediates in the chemical industry and are suited for producing a variety of industrial chemicals. Typically the nitroaromatic compositions are hydrogenated to form the aromatic amine which then can be used in preparing dyestuffs, or as an intermediate in the urethane industry. Examples of widely used nitroaromatics which are subsequently hydrogenated to the amine include nitrobenzene, dinitrotoluene, and dinitroxylene. These compositions when hydrogenated form aniline, toluenediamine, and xylenediamine, respectively. The former has wide utility for the dyestuff industry while the latter diamines are used as intermediates in the production of diisocyanates for urethane manufacture.
Numerous processes have been developed for effecting nitration of aromatic hydrocarbons; some have focused on the synthesis of mononitroaromatics while others are focused on the production of dinitro or trinitro aromatic compositions. Nitration of aromatic compositions have typically been done by the mixed acid technique, i.e. a mixture of nitric acid and sulfuric acid, although nitration of aromatics has also been affected utilizing nitric acid alone or mixtures of nitrogen oxides and sulfuric acid. Representative patents illustrating some of the nitration techniques are as follows:
U.S. Pat. Nos. 2,362,743, 2,739,174 and 3,780,116 disclose processes for the nitration of aromatic hydrocarbons using nitric acid as the sole nitrating medium. The '743 patent uses a two-stage nitration process to form dinitrotoluene in the first stage, toluene is nitrated with 60-75% nitric acid at temperatures about 75.degree.-80.degree. C. and then dinitrated with 90-100% nitric acid at the same temperature. In the '174 patent benzene, toluene, and xylene were nitrated using 70% nitric acid at temperatures of from about 110.degree.-120.degree. C. In the process, a liquid reaction mixture comprising water, nitric acid and nitrated hydrocarbon was withdrawn from the reactor and the nitric acid separated from the water nitrated hydrocarbon azeotrope via distillation. The '116 patent used approximately 40% nitric acid as the nitrating medium for benzene and toluene and the process involved bubbling hydrocarbon vapor through the nitric acid medium at temperatures of from about 50.degree.-100.degree. C. A nitrobenzene-nitric acid mixture is withdrawn from the reactor and the mixture separated by decavitation. Nitric acid and unreacted benzene and water are removed as vapor with the benzene being separated and returned.
By and large the technique for nitrating aromatic hydrocarbons such as benzene, toluene, zylene, napthalene, anthraquinone has been the mixed acid technique. U.S. Pat. Nos. 2,256,999; 2,370,558; 2,773,911; 2,849,497; 3,434,802; 4,021,498; and 4,112,005 disclose variations in the mixed acid technique for producing nitroaromatic compositions and particularly, mono and dinitroaromatic compositions. The mixed acid technique is preferred in the manufacture of nitroaromatics since the concentration of the nitronium ion, which is the nitrating agent, is much lower in nitric acid alone than in the mixed acid. In the mononitration of aromatics, the aromatic hydrocarbons are contacted with a nitric acid/sulfuric acid mixture, the nitric acid concentration typically being about 20-70% by volume or more dilute than in dinitration reaction. The sulfuric acid typically used in 80-98% concentration.
The '005 patent mentioned above discloses preparing the mononitroaromatic compounds by nitrating a reactive aromatic compound in the absence of sulfuric acid until mononitration is complete, the nitration being carried out at 40-68% by weight nitric acid.
Dinitroaromatics, e.g. dinitroxylene and particularly dinitrotoluene have been typically produced by using highly concentrated nitric acid compositions or the mixed acid technique and U.S. Pat. Nos. 2,362,743; 2,934,571; and 3,092,671 are representative. The '743 patent effects dinitration of toluene in the absence of sulfuric acid. The mononitration is carried out with 70% nitric acid, while the dinitration is carried out using 98% nitric acid at temperatures of about 70.degree.-80.degree. C. High mole ratios of acid. e.g. 2-5:1 moles nitric acid per mole hydrocarbon, are required. The '571 patent discloses the nitration of various aromatics such as benzene. nitrobenzene, halogen-substituted benzenes, and so forth by the mixed acid technique. In that process a mixture of fuming nitric acid and fuming sulfuric acid are reacted with the aromatic hydrocarbon at temperatures of 50.degree.-60.degree. C.
Commercially, the nitration of toluene to form dinitrotoluene is done in a two-step process Wherein mononitrotoluene is formed in a first stage, the water of reaction and spent acid being removed from the mononitrobenzene reaction product and then the mononitrobenzene charged to the dinitrator for subsequent nitration.
The hydrogenation of nitroaromatics to the amine, e.g. nitrobenzene to aniline and dinitrotoluene to toluenediamine, usually has been carried out by effecting the hydrogenation in aqueous phase over a hydrogenation/dehydrogenation catalyst such as Raney nickel. Normally in the hydrogenation process, the nitrated product is purified and removed of acidic material and alkaline material which act as catalyst poisons in the hydrogenation reaction. U.S. Pat. No. 4,224,249 discloses such a process for this hydrogenation.
Recently it has been reported (U.S. Pat. No. 4,185,036) that mixed nitroaromatic compositions can be hydrogenated under appropriate conditions to form aromatic amines with less by-product tars at higher rates and higher yields than are achieved with prior art hydrogenation processes. More particularly, the patent discloses that a mixture of aromatic compounds can be selectively hydrogenated to the amine, the mixture containing at least 25% of a mononitronon-aminoaromatic compound and at least 25% of a dinitro or a mononitro-amino compound. Examples of reactants which are suited for the selective cohydrogenation include o-nitrotoluene, o-nitroaniline, mononitrotoluene, dinitrobenzene, dinitrotoluene and other nitro aromatics.
In view of the ability to selectively hydrogenate mixed aromatic compounds while reducing the amount of tar formation during the hydrogenation of such compounds, a need was created for feeds of mixed nitroaromatic compositions. The need was originally satisfied by mixing a purified mononitronon-amino compound with a purified dinitro compound or a nitroamino aromatic and the hydrogenation effected of that mixture. Although this process is well-suited to the creation of mixed nitroaromatic compounds, there were problems associated in the preparation of the purified nitroaromatics due to the reaction conditions and separation techniques.